Optimize fetching last change data

Avoid using exception, we first figure out if object exists and then
fetch it including related user record (which we will need in most
cases anyway).

Issue #861
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/translation.py

@@ -679,11 +679,15 @@ class Translation(models.Model, URLMixin, PercentMixin, LoggerMixin):
 
     @property
     def last_change_obj(self):
+        """Cached getter for last content change."""
         if not self._last_change_obj_valid:
-            try:
-                self._last_change_obj = self.change_set.content()[0]
-            except IndexError:
+            changes = self.change_set.content()
+
+            if changes.exists():
+                self._last_change_obj = changes.select_related('author')[0]
+            else:
                 self._last_change_obj = None
+
             self._last_change_obj_valid = True
 
         return self._last_change_obj